As is commonly known, an aluminum material has a lower hardness than steel and the like, and very easily seizes and wears away when it slides against steel and the like. Therefore, various surface treatments of aluminum materials using metal plating, spray forming, and anodizing have been studied and practiced. These surface treatments are mainly to form an aluminum oxide layer on the surface of an aluminum material. Although nitriding has been attempted, nitride layers formed on the surface are thin, and satisfactory surface nitrided aluminum base materials have not been obtained. This is supposed to be because an aluminum material is a metal which is very active and easily oxidized, and always has some oxide layer on the surface.
The present inventors proposed a nitriding method comprising contacting at least part of the surface of an aluminum material with a nitriding auxiliary agent including aluminum powder, and with keeping this state, nitriding the surface of the aluminum material by an atmospheric gas substantially comprising nitrogen gas at a nitriding temperature which is equal to or lower than a melting point of the aluminum material in the publication of Japanese Unexamined Patent Publication (KOKAI) No.H7-166321. In this method, when aluminum powder used as a nitriding auxiliary agent is contacted with nitrogen gas at a predetermined temperature, the aluminum powder is nitrided in itself, and at this time, nascent nitrogen (N*) generates and diffuses into the interior of the aluminum material, thereby forming a nitride layer.
It is desirable that an aluminum material to be nitrided or an aluminum material constituting a nitriding auxiliary agent contains magnesium, because nitriding is promoted, nitriding speed is increased, and a thicker nitride layer is formed. This is supposed to be because magnesium serves as an oxygen getter.
In regard to aluminum, although pure aluminum is used alone, aluminum alloys containing copper, zinc, silicon, magnesium or the like in addition to aluminum are used industrially. In particular, as aluminum alloys used as castings, aluminum-silicon alloys are often used because of excellent castability (fluidity).
On the other hand, in the aforementioned surface nitriding method of an aluminum material, in the case where aluminum alloy powder containing magnesium, which has a strong nitriding power, is used as a nitriding auxiliary agent, and nitriding treatment is applied to an aluminum alloy material by using pure nitrogen gas at a nitriding temperature of 500 to 550.degree.C. for five to ten hours, a nitride layer of 50 to 300 .mu.m is obtained. In the case where an aluminum alloy material to be nitrided contains silicon, however, even if nitriding treatment is applied under the same nitriding conditions, the thickness of an obtained nitride layer is about one fifth to one tenth of that in the case where an aluminum alloy material containing no silicon is used.